<div>
        <asp:LinkButton ID="Bold" Text="B" runat="server"   OnClick="Bold_Click"></asp:LinkButton>
        <asp:LinkButton ID="Italic" Text="I" runat="server" OnClick="Italic_Click"></asp:LinkButton>
        <asp:LinkButton ID="UnderLine" Text="U" runat="server" OnClick="UnderLine_Click"></asp:LinkButton>
</div>
<div>
        <asp:TextBox ID="txt" runat="server" TextMode="multiline"></asp:TextBox>
</div>

protected void Bold_Click(object sender, EventArgs e)
{
        txt.Font.Bold = true;
}
protected void Italic_Click(object sender, EventArgs e)
{
        txt.Font.Italic = true;
}
protected void UnderLine_Click(object sender, EventArgs e)
{
        txt.Font.Underline = true;
}